1. Field of the Invention
The present invention relates to a line replaceable module LRM for a digital avionics system, and more particularly to a modular LRM configured from self-contained mini-boards, for example, two to four mini-boards, with increased functionality adapted to interface with the digital avionics system by way of a high contact density backplane connector.
2. Description of the Prior Art
Avionic control systems aboard aircraft are implemented by way of modules that are adapted to be connected to the aircraft data bus. Such modules are known as line replaceable modules (LRM). For example, the Boeing B-777 Airplane Information Management System (AIMS) utilizes a total of eleven LRMs connected to common chassis. Due to the limited space available on aircraft, generally only one or two chassis' are permitted per aircraft. Each chassis may include two power LRMs, each connected to different power buses; four I/O LRMs; three central processing modules (CPM) LRMs; an autothrottle LRM; and a communication LRM. The various LRMs within the chassis are used for various functions, including flight management, electronic flight instrument systems, engine indicating and crew alerting system display management.
Various bus architectures are known for interconnecting the LRMs. In civil aircraft, the LRMs within each chassis are known to be connected to what is known as an ARINC 659 backplane data bus, which operates at about 30 MBIT/S over either a twisted wire pair or fiberoptic cables.
Due to the limited space aboard an aircraft, the form factor of such LRMs is specified by various standards. For example, a MIL-STD-28787 standard describes a number of standard configurations and sizes for electronic modules, including LRMs. The aforementioned standard specifies a Standard Electronic Module-Size E (SEM-E) form factor for an LRM defined as a module 5.88" high and 6.4" deep. The width of the SEM-E module can vary in 0.1" increments from about 0.28" to 0.58". The dimensional constraints of the SEM-E LRM limits contact density to about 400 or less. Unfortunately, with the ever increasing complexity of avionics, higher contact densities are required.
Known LRMs include two to four printed circuit boards (PCB) for example, up to a maximum size of 5".times.5" for carrying various components to perform the specified function as discussed above. Each PCB is formed with an edge connector along one edge for electrically interfacing the PCB to a backplane data bus within the LRM chassis. In applications where contact densities of more than 400 are required, one known approach is to provide interconnections between the PCBs, as well as reduced spacing between contacts. As such, known LRM's which must meet the SEM-E form factor utilize flexible connectors and/or cross-overs to provide interfaces between the PCBs. Due to the different contact lengths and close spacing required in such applications, electrical performance is known to be degraded in such applications as a result of the impedance variability and cross-talk between contacts.
There are other problems associated with known LRMs. For example, fault detection and fault isolation capabilities are required down to the component level. As such, in applications where increased contact densities are required, the fault detection and fault isolation requirements result in relatively complex boards increasing the cost and complicating the maintenance of such boards. Moreover, known SEM-E modules are designed and fabricated by single suppliers with virtually no integration capability between suppliers. In addition, the current costs of such modules is in the range of $15,000-$20,000. Due to such a high cost, such modules are not disposable and are known to result in relatively expensive fault diagnostics and repair when problems are detected. Thus, there is a need for reduced cost modular LRM which enables defective modules to be discarded and which enables all PCBs in the module to interface by way of the backplane database rather than the interboard connectors.